How To Read Sidewall Markings

There is a lot of information on the sidewall of a tire. Typically, you'll find UTQG ratings for treadwear, traction and temperature, the size of the tire, the load rating index number with a speed rating index, the construction type (bias or radial), the D.O.T. (Dept. of Transportation) compliance code, construction details, and of course, the make and model of the tire. On some tires used as original equipment, you may also find a marking that indicates its OE status. Porsche uses an N-0 or N-1 designation, BMW uses a star on some O.E. tires and General Motors uses a "TPC" code. Light Truck tires are sometimes marked with an LT for "Light Truck" before the size, passenger tires are often marked with the letter P for "Passenger" before the size. Passenger tires of the same size with or without the P are virtually interchangable.

Tire Dimension Calculator

The first number is the width of the tire in millimeters, measured from sidewall to sidewall. To convert to inches, divide by 25.4 In the example above, the width is 185mm or 7.28".

The second number is the aspect ratio. This is a ratio of sidewall height to width. In the example above, the tire is 7.28" wide, multiply that by the aspect ratio to find the height of one sidewall. In this case, 185x0.60=111mm or 7.28"x0.60=4.36".

The last number is the diameter of the wheel in inches.

To figure the outside diameter of a tire, take the sidewall height and multiply by 2, (remember that the diameter is made up of 2 sidewalls, the one above the wheel, and the one below the wheel) and add the diameter of the wheel to get your answer.

Example...185/60R14 85H or 185/60HR14

185mm x .60=111mm x 2=222mm + 355.6mm(14")= 577.6mm or 22.74"

Basic Tire Construction

Radial Tires Versus Diagonal (Bias) Tires

Radial Tires

The body cords run across the tire perpendicular to the beads.  Radial tires have belt plies which run nearly circumferentially around the tire, under the tread, to stabilize the tread, to strengthen the tread area and add flexibility to the sidewall.  By restricting tread movement during contact with the road, The belt plies improve tread life.

Diagonal (Bias) Tires

These tires have two, four, or more body plies which cross at an angle to the centerline of the tread and which strengthen both the sidewall and the tread area.

Mixed Use Of Tire Construction

Tires of different constructions have different performance and handling characteristics.

 

Due to this fact, be cautioned that the following instructions MUST be followed:

 

Never mix tires of different size or construction on the same axle.

 

Radial ply tires may be mixed with bias ply or bias belted tires, provided the radials are mounted in pairs on the rear axle.

Steering Characteristics

Neutral Steer:  Theoretically, the ideal steering condition.

 

Understeer:  The condition present when a vehicle is less responsive turning a corner or making a lane change.  This characteristic is more desirable than oversteer because the driver remains in better control of the vehicle.

 

Oversteer:  The condition present when a vehicle tends to turn a corner or change lanes more sharply than the driver expects. If a vehicle is traveling at high speeds, this condition can be dangerous, as the vehicle may easily spin out of control.  Never place radial tires on the front axle with bias on the rear.